Piston mode lamb wave resonators

ABSTRACT

Piston mode Lamb wave resonators are disclosed. A piston mode Lamb wave resonator can include a piezoelectric layer, such as an aluminum nitride layer, and an interdigital transducer on the piezoelectric layer. The piston mode Lamb wave resonator has an active region and a border region, in which the border region has a velocity with a lower magnitude than a velocity of the active region. The border region can suppress a transverse mode.

CROSS REFERENCE TO PRIORITY APPLICATION

This application claims the benefit of priority of U.S. ProvisionalPatent Application No. 62/553,747, filed Sep. 1, 2017 and titled “PISTONMODE LAMB WAVE RESONATORS,” the disclosure of which is herebyincorporated by reference in its entirety herein.

BACKGROUND Technical Field

Embodiments of this disclosure relate to Lamb wave resonators.

Description of Related Technology

Piezoelectric microelectromechanical systems (MEMS) resonators can beused in radio frequency systems. Piezoelectric MEMS resonators canprocess electrical signals using mechanically vibrating structures. Somepiezoelectric MEMS resonators are aluminum nitride (AlN) Lamb waveresonators.

Acoustic wave filters can include Lamb wave resonators. Acoustic wavefilters can filter radio frequency signals in radio frequency electronicsystems. For instance, filters in a radio frequency front end of amobile phone can include acoustic wave filters. Multiple acoustic wavefilters can be arranged as a multiplexer, such as a duplexer.

SUMMARY OF CERTAIN INVENTIVE ASPECTS

The innovations described in the claims each have several aspects, nosingle one of which is solely responsible for its desirable attributes.Without limiting the scope of the claims, some prominent features ofthis disclosure will now be briefly described.

One aspect of this disclosure is a piston mode Lamb wave resonator withtransverse mode suppression. The piston mode Lamb wave resonatorincludes a piezoelectric layer and an interdigital transducer, in whichthe interdigital transducer is on the piezoelectric layer. The pistonmode Lamb wave resonator has an active region and a border region. Theborder region has a first velocity with a lower magnitude than a secondvelocity of the active region. The border region is configured tosuppress a transverse mode. The piston mode Lamb wave resonator isconfigured to generate a Lamb wave.

The interdigital transducer can include a bus bar and a plurality offingers extending from the bus bar, in which each of the fingers has anend portion opposite the bus bar. The end portions of the fingers caninclude metal that is wider than other portions of the respectivefingers. The interdigital transducer can include a second bus bar havinga lower metal coverage ratio adjacent the end portions than in otherportions of the second bus bar. An oxide can be included over the endportions of the fingers. Alternatively or additionally, silicon nitridecan be included over a portion of the interdigital transducer, and theend portions being free from the silicon nitride.

The border region can have a larger metal coverage ratio than the activeregion. Alternatively or additionally, the border region can have alarger metal coverage ratio than an inactive region of the piston modeLamb wave resonator.

The piston mode Lamb wave resonator can include reflective gratings onopposing sides of the interdigital transducer. Alternatively, the pistonmode Lamb wave resonator can have free edges.

The piezoelectric layer can be an aluminum nitride layer. The aluminumnitride layer can be on a silicon layer. The piston mode Lamb waveresonator can include an air cavity on an opposite side of thepiezoelectric layer than the interdigital transducer. Alternatively, thepiston mode Lamb wave resonator can include an acoustic mirror on anopposite side of the piezoelectric layer than the interdigitaltransducer.

Another aspect of this disclosure is an acoustic wave filter with apiston mode Lamb wave resonator. The acoustic wave filter includes aninput node configured to receive a radio frequency signal, an outputnode, and a piston mode Lamb wave resonator coupled between the inputnode and the output node. The piston mode Lamb wave resonator has anactive region and a border region. The border region has a firstvelocity with a lower magnitude than a second velocity of the activeregion. The border region is configured to suppress a transverse mode.The acoustic wave filter is configured to filter the radio frequencysignal.

The border region can have a larger metal coverage ratio than the activeregion. The Lamb wave resonator can include an interdigital transducerthat includes a bus bar and fingers extending from the bus bar, in whicha finger of the fingers includes an end portion opposite the bus barthat has metal that is wider than other portions of the finger. The Lambwave resonator can include an interdigital transducer and a layer overthe interdigital transducer that contributes to the first velocityhaving the lower magnitude than the second velocity. The Lamb waveresonator can include an aluminum nitride layer.

Another aspect of this disclosure is a method of filtering a radiofrequency signal using a piston mode Lamb wave resonator. The methodincludes providing the radio frequency signal to a filter that includesthe piston mode Lamb wave resonator, in which the Lamb wave resonatorincluding an aluminum nitride piezoelectric substrate. The methodfurther includes filtering the radio frequency signal with the filter.The filter includes suppressing a transverse mode using a border regionof the piston mode Lamb wave resonator, in which the border region has afirst velocity with a lower magnitude than a second velocity of anactive region of the piston mode Lamb wave resonator.

A metal layout of an interdigital transducer of the piston mode Lambwave resonator can contribute to the first velocity having the lowermagnitude than the second velocity. Alternatively or additionally, alayer over at least a portion of the interdigital transducer cancontribute to the first velocity having the lower magnitude than thesecond velocity.

For purposes of summarizing the disclosure, certain aspects, advantagesand novel features of the innovations have been described herein. It isto be understood that not necessarily all such advantages may beachieved in accordance with any particular embodiment. Thus, theinnovations may be embodied or carried out in a manner that achieves oroptimizes one advantage or group of advantages as taught herein withoutnecessarily achieving other advantages as may be taught or suggestedherein.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of this disclosure will now be described, by way ofnon-limiting example, with reference to the accompanying drawings.

FIG. 1 is a graph that includes dispersion curves for the first fourLamb wave modes of a Lamb Wave resonator and the desired border regionEigen-frequencies for S₀ and S₁ modes.

FIG. 2 is a graph that illustrates spectra of the theoretical transversespurious resonances as a function of aperture width in lines and finiteelement method (FEM) simulated cases in dots.

FIG. 3 is a graph that illustrates a slowness curve of the transducerregion, gap region, and bus bar region for the S0 mode of a Lamb waveresonators in aluminum nitride.

FIG. 4 is a graph that illustrates a comparison of a FEM simulatedconductance with different normalized gap widths.

FIGS. 5A and 5B compare a velocity profile and displacement profiles ofa traditional Lamb wave resonator and a piston mode Lamb wave resonator.FIG. 5A illustrates an interdigital transducer (IDT) of traditional Lambwave resonator and a corresponding velocity profile and displacementprofiles. FIG. 5B illustrates an IDT of a piston mode Lamb waveresonator according to an embodiment and a corresponding velocityprofile and displacement profiles.

FIG. 6 is a graph that includes a comparison of the FEM simulatedfrequency response for S0 mode Lamb wave resonators with two types ofhammer-head IDTs for piston mode according to embodiments and atraditional IDT.

FIG. 7 is a graph that includes a comparison of the FEM simulatedfrequency response for type I S₁ mode Lamb wave resonators with twotypes of hammer-head IDTs for piston mode according to embodiments and atraditional IDT.

FIG. 8 is a graph of a FEM simulated admittance for a piston mode devicewith a border region width that is larger than typically desired.

FIG. 9A is a diagram cross section of a Lamb wave resonator according toan embodiment.

FIG. 9B is a diagram of cross section of a Lamb wave resonator accordingto another embodiment.

FIGS. 10A to 10J are diagrams of IDTs of piston mode Lamb waveresonators according to various embodiments. FIG. 10A illustrates an IDTwith fingers having hammer head shaped end portions. FIG. 10Billustrates an IDT with thicker portions for both border regions of eachfinger. FIG. 10C illustrates an IDT with fingers having hammer headshaped end portions and bus bars having extension portions adjacent tothe end portions of the fingers. FIG. 10D illustrates an IDT withthicker end portions on border regions of each finger and bus barshaving extension portions adjacent to end portions of the fingers. FIG.10E illustrates an IDT with fingers having thicker end portions andthicker regions extending from a bas bar toward an active region. FIG.10F illustrates an IDT with bus bars having holes. FIG. 10G illustratesanother IDT with bus bars having holes. FIG. 10H illustrates an IDT withfingers having thicker metal on both border regions. FIG. 10Iillustrates an IDT with fingers having an oxide over border regions.FIG. 10J illustrates an IDT with fingers having silicon nitride over anactive region and border regions free from silicon nitride.

FIGS. 11A to 11F are diagrams of cross sections of Lamb wave resonatorswith gratings. FIG. 11A illustrates a Lamb wave resonator with agrounded electrode. FIG. 11B illustrates a Lamb wave resonator with afloating electrode. FIG. 11C illustrates a Lamb wave resonator withoutan electrode on a side of a piezoelectric layer that opposes aninterdigital transducer (IDT) electrode. FIG. 11D illustrates anotherLamb wave resonator. FIG. 11E illustrates another Lamb wave resonator.FIG. 11F illustrates another Lamb wave resonator.

FIGS. 12A to 12F are diagrams of cross sections of Lamb wave resonatorswith free edges. FIG. 12A illustrates a Lamb wave resonator with agrounded electrode. FIG. 12B illustrates a Lamb wave resonator with afloating electrode. FIG. 12C illustrates a Lamb wave resonator withoutan electrode on a side of a piezoelectric layer that opposes an IDTelectrode. FIG. 12D illustrates another Lamb wave resonator. FIG. 12Eillustrates another Lamb wave resonator. FIG. 12F illustrates anotherLamb wave resonator.

FIG. 13 is a schematic block diagram of a piston mode Lamb waveresonator and complementary metal oxide semiconductor (CMOS) circuitryon a semiconductor die.

FIG. 14 is a schematic block diagram of an oscillator that includes apiston mode Lamb wave resonator.

FIG. 15 is a schematic block diagram of a sensor that includes a pistonmode Lamb wave resonator.

FIG. 16 is a schematic block diagram of a filter that includes a pistonmode Lamb wave resonator.

FIG. 17 is a schematic block diagram of a module that includes a filterwith a piston mode Lamb wave resonator.

FIG. 18 is a schematic block diagram of a wireless communication devicethat includes a filter with a piston mode Lamb wave resonator.

DETAILED DESCRIPTION OF CERTAIN EMBODIMENTS

The following description of certain embodiments presents variousdescriptions of specific embodiments. However, the innovations describedherein can be embodied in a multitude of different ways, for example, asdefined and covered by the claims. In this description, reference ismade to the drawings where like reference numerals can indicateidentical or functionally similar elements. It will be understood thatelements illustrated in the figures are not necessarily drawn to scale.Moreover, it will be understood that certain embodiments can includemore elements than illustrated in a drawing and/or a subset of theelements illustrated in a drawing. Further, some embodiments canincorporate any suitable combination of features from two or moredrawings.

Piezoelectric microelectromechanical systems (MEMS) resonators offerfascinating prospects for frequency selection, control, and sensingapplications thanks to their relatively small size and relatively lowresonance impedance. Among various piezoelectric MEMS resonators,aluminum nitride (AlN) Lamb wave resonators are capturing attentionsince they enjoy advantages of both surface acoustic wave (SAW) devicesand film bulk acoustic resonators (FBARs). Such Lamb wave resonators canhave multi-frequency and complementary metal oxide semiconductor (CMOS)compatibility. For all Lamb wave modes propagating in an aluminumnitride plate, the lowest order symmetric (S₀) and first order symmetric(S₁) modes stand out for their transduction efficiency.

A Lamb wave resonator can combine features of a SAW resonator and a BAWresonator. A Lamb wave resonator typically includes an interdigitaltransducer (IDT) electrode similar to a SAW resonator. Accordingly, thefrequency of the Lamb wave resonator can be lithographically defined. ALamb wave resonator can achieve a relatively high quality factor (Q) anda relatively high phase velocity like a BAW filter (e.g., due to asuspended structure). A Lamb wave resonator that includes an aluminumnitride piezoelectric layer can be relatively easy to integrate withother circuits, for example, because aluminum nitride process technologycan be compatible with complementary metal oxide semiconductor (CMOS)process technology. Aluminum nitride Lamb wave resonators can overcome arelatively low resonance frequency limitation and integration challengeassociated with SAW resonators and also overcome multiple frequencycapability challenges associated with BAW resonators. Aluminum nitrideLamb wave resonators can also be desirable due to their relatively smallsize.

In general, high quality factor (Q), large effective electromechanicalcoupling coefficient (k² _(eff)), high frequency ability, and spuriousfree are 4 significant aspects for micro resonators to enable low-lossfilters, stable oscillators, and sensitive sensors. While the lowestorder symmetric (S₀) and first order symmetric (S₁) Lamb wave resonatorsexcellently address the high (Q), large (k² _(eff)), and high frequencyability, they exhibit can strong affinity toward multimode behavioralong with their transduction efficiency. Lamb wave resonators can haverelatively strong transverse mode in and/or near a pass band. Thepresence of the relatively strong transverse modes can hinder theaccuracy and/or stability of oscillators and sensors, as well as hurtthe performance of acoustic filters by creating relatively severepassband ripples and potentially limiting the rejection.

Apodization is a solution for transverse modes in the interdigitaltraducer electrode (IDT)-excited devices by varying the resonance cavitylength to smooth out the effect of transverse mode on the electricalresponse. Its incomplete transduction from the electrical field to mainmode resonance can result in substantial drawbacks, such as reduction ofthe transduction efficiency (e.g., degraded k²) and additional losses(e.g., degraded Q) for both SAW filters and Lamb wave resonators.

A technical solution for suppressing transverse modes is to create aborder region with a different frequency from active region according tothe mode dispersion characteristic. This can be referred to as a “pistonmode.” A piston mode can be obtained to cancel out the transverse wavevector in a lateral direction without significantly degrading the k² orQ. By including a relatively small border region with a slow velocity onthe edge of the acoustic aperture of a Lamb wave resonator, apropagating mode can have a zero (or approximately zero) transverse wavevector in the active aperture. The transverse wave vector can be real inthe border region and imaginary on a gap region. A piston mode Lamb waveresonator can have even order modes that have a multiple of full wavelengths in the active region, which should not significantly couple toelectrical domain. A piston mode Lamb wave resonator have maintaintransduction without introducing acoustic loss compared to apodization.

Most Lamb wave modes in aluminum nitride show a positive slope in thedispersion branch, so that a border region of lower eigenresonancefrequency may be desired for spurious mode suppression. Dispersioncalculations and finite element method (FEM) simulations indicate that,by changing transducer layout, the guiding can be improved and a pistonmode can be obtained for the type I Lamb wave modes.

FIG. 1 is a graph that includes dispersion curves for the first fourLamb wave modes of a Lamb Wave resonator and the desired border regionEigen-frequencies for S₀ and S₁ modes. The S₀ mode exhibits positiveslope through all wave numbers in the illustrated range. This indicatesa positive group velocity (v_(g)) and type I dispersion. The S₁ mode hasa negative group velocity at h_(A1N)/λ<0.3 corresponding to type IIdispersion and positive slope at h_(A1N)/λ>0.3 corresponding to type Idispersion.

To operate in piston mode, for resonances of type I dispersion, thefrequency of the border region should be lower than that of the activearea. This is similar to the typical case for ZnO FBAR and SAW filters(non-dispersive Rayleigh wave or SH wave has positive dispersion slopeof 2π·v_(p)). On the contrary, for resonances of type II dispersion, thefrequency of the border region should be higher than that of the activearea to operate in piston mode. This is typically the case for AlN FBAR.

However, for the LWR employing S₁ mode in the type II region, thetransverse modes are not typically strongly excited, mostly below fs.Some other spurious modes, such as the inter-reflection from IDT fingersdue to negative velocity, are typically much stronger and more ofconcern. Accordingly, the transverse modes and the design of piston modefor S0 mode and type I S1 mode are emphasized herein.

Similar to FBAR and SAW resonators, the odd transverse modes arenaturally cancelled in the electrical response of an LWR. This is shownin the left column of FIG. 5.

The IDT aperture and transverse mode order can directly determine thewave number in a lateral direction βy as indicated by Equation 1.

$\begin{matrix}{\beta_{y} = \frac{\pi \cdot ( {n + 1} )}{W_{a}}} & ( {{Equation}\mspace{14mu} 1} )\end{matrix}$

In Equation 1, w_(a) represents the active region width or aperturewidth and n represents the transverse mode order. The wave number inpropagation direction βx can remain unchanged as reflected in Equation2.

$\begin{matrix}{\beta_{x} = {\frac{2\pi}{\lambda}.}} & ( {{Equation}\mspace{14mu} 2} )\end{matrix}$

Using orthogonal wave number vector superposition, the wave number valueβn and frequency fn of each transverse mode can be estimated usingEquations 3 and 4, respectively.

$\begin{matrix}{{\beta_{n,{trans}} = {2\pi\sqrt{( \frac{n + 1}{2 \cdot w_{a}} )^{2} + ( \frac{1}{\lambda} )^{2}}}},} & ( {{Equation}\mspace{14mu} 3} ) \\{f_{n,{trans}} = {\frac{v_{p}}{\lambda}{\sqrt{1 + ( \frac{( {n + 1} )/2}{w_{a}/\lambda} )^{2}}.}}} & ( {{Equation}\mspace{14mu} 4} )\end{matrix}$

FIG. 2 is a graph that illustrates spectra of the theoretical transversespurious resonances as a function of aperture width in lines and finiteelement method (FEM) simulated cases in dots. FIG. 2 shows thetheoretical curves from Equation 4 and FEM simulated cases. The longerthe normalized active region w_(A)/λ, the more transverse mode should bein passband, but with smaller amplitude because of inter energydissipation. The FEM simulated frequencies are lower than theoretical,which can be because the inactive regions such as the bus bar and gapare taken into account. A longer aperture can bring in more transversemodes and k² _(eff) can degrade for too-short aperture as f_(s) shiftedto higher.

FIG. 3 is a graph that illustrates a slowness curve of the transducerregion, gap region, and bus bar region for the S0 mode of a Lamb waveresonators in aluminum nitride. The slowness curve corresponds to a casewhere the height of the aluminum nitride layer is 0.1λ, wherein λ is awavelength of the Lamb wave resonator. The degree θ monotonicallyincreases with frequency for the transducer, bus bar, and gap regions inFIG. 3. This indicates that t the S0 Lamb wave exhibits a convexslowness curve feature in all regions.

Usually for a wave with a convex slowness curve to be trapped, a fasterregion is desired at the lateral end to guide the wave. For temperaturecompensated surface acoustic wave (TCSAW) resonators on SiO₂/LiNbO₃, thefaster gap region can help trap energy. For Quartz resonators, thefaster bus bar region performing wave guiding. However, for the S₀ modeLamb wave mode, the gap region exhibits too of a high phase velocity tomake the wave guiding function as desired at the transducer/gapinterface. Also, as the bus bar region has phase velocity close to thetransducer region, the wave guiding can be close to the bus bar/gapinterface. Accordingly, by modifying the gap region width w_(g), theenergy trapping should not have much impact. Unlike the TCSAW that has alarger Q for wider gap, the energy loss of S₀ mode Lamb wave resonatoris not typically dependent on the gap width. Rather, the gap width canhave direct impact on the transverse mode amplitude for an S₀ mode Lambwave resonator.

FIG. 4 is a graph that illustrates a comparison of a FEM simulatedconductance with different normalized gap widths w_(g). FIG. 4 depictsthe simulated conductance of S0 mode Lamb wave resonators with differentgap widths using FEM. When wg/λ is 2, the transverse mode level canbecome minimized. For slightly larger wg, the effective active regiongot stretched a little bit so that each transverse mode got smaller Qand lower frequency, especially the first two modes 2nd and 4th. Whenthe gap becomes too large, the increase in the k²eff should overcome theQ decrease, resulting in larger amplitude.

For the type I modes, by adding a relatively small border region with arelatively slow velocity on the edge of the acoustic aperture, apropagating mode can have a zero transverse wave vector in the activeaperture. The transverse wave vector is real in the border region andimaginary on the gap region. One embodiment of the border regionincludes using a larger metal coverage ratio electrode in the borderregion. This can be a “hammer head” shape in the border region in planview. FIG. 5B illustrates an example of an interdigital transducer (IDT)of a Lamb wave resonator with such an electrode.

FIGS. 5A and 5B compare a velocity profile and displacement profiles ofa traditional Lamb wave resonator and a piston mode Lamb wave resonator.These profiles correspond to aluminum nitride Lamb wave resonators. FIG.5A illustrates an IDT 10 of a traditional Lamb wave resonator and acorresponding velocity profile and displacement profiles. FIG. 5Billustrates an IDT 20 of a piston mode Lamb wave resonator and acorresponding velocity profile and displacement profiles.

The IDT 10 illustrated in FIG. 5A includes a first bus bar 12, firstfingers 14A and 14B extending from the first bus bar 12, a second busbar 16, and second fingers 18A and 18B extending from the second bus bar16. In FIG. 5A, the first fingers 14A and 14B have the same width alonga length wa of an active region and a length wg of a gap region.Similarly, the second fingers 18A and 18B have the same width along alength wa of an active region and a length wg of a gap region in FIG.5A.

The IDT 20 illustrated in FIG. 5B is an example of an IDT for a pistonmode Lamb wave resonator according to an embodiment. The IDT 20 includesa first bus bar 12, first fingers extending from the first bus bar 12, asecond bus bar 16, and second fingers extending from the second bus bar16. The IDT 20 can be implemented with any suitable number of fingers.

Each of the first fingers includes a body portion 24A or 24B extendingfrom the first bus par 12 in an active region and an end portion 25A or25B in a border region opposite the first bus bar 12. The end portions25A and 25B include thicker metal than the body potions 24A and 24B,respectively, of the first fingers. This can result in a slower velocityin the border region than in the active region. Similarly, each of thesecond fingers includes a body portion 28A or 28B extending from thesecond bus par 16 in the active region and an end portion 29A or 29B ina border region opposite the second bus bar 16. The end portions 29A and29B include thicker metal than the body potions 28A and 28B,respectively, of the second fingers. This can result in a slowervelocity in the border region than in the active region.

As illustrated in FIG. 5B, the first fingers are thicker in a borderregion along a length wb than in the active region along length wa. Thefirst fingers are also thicker in the border region than in a gap regionalong length wg in the IDT 20. Similarly, in the IDT 20, the secondfingers are thicker in a border region along a length wb than in aremainder of the active region along length wa. The second fingers arealso thicker in the border region than in a gap region along length wgin the IDT 20. The fingers include respective end portions 24A, 24B,29A, and 29B in border regions. The end portions 24A, 24B, 29A, and 29Bhave a hammer head shape in border regions in FIG. 5B.

FIGS. 5A and 5B compare the velocity profiles of a traditional Lamb waveresonator with the IDT 10 and a piston mode Lamb wave resonator with theIDT 20. As shown in FIGS. 5A and 5B, the Lamb wave resonator thatincludes IDT 20 has a reduced velocity in border regions compared to thegap regions and the active region. In FIG. 5B, the reduced velocity inthe border region is caused by the end portions 25-1, 25-2, 25-3, and25-4 of the IDT 20.

FIGS. 5A and 5B also compare the displacement profiles u_(y,n) of thenth transverse mode for n=0, 1, 2, 3, and 4 in a traditional Lamb waveresonator with the IDT 10 and displacement profiles u_(y,n) of the apiston mode Lamb wave resonator with the IDT 20. As shown in FIGS. 5Aand 5B, the velocity profiles for even order modes have a slope with agreater magnitude corresponding to the border region of the IDT 20 ofFIG. 5B than in the corresponding portion of the active region of theIDT 10 of FIG. 5A. For the traditional Lamb wave resonator with the IDT10, approximately an extra half wave lengths fits to the active regionfor even modes. This can lead to non-vanishing coupling. In the pistonmode Lamb wave resonator with the IDT 20, the even order modes that havea multiple of full wave lengths in the active region should notmeaningfully couple to the electrical domain. Accordingly, FIGS. 5A and5B indicate that the IDT 20 can suppress transverse modes moreeffectively than the IDT 10.

FIGS. 6 and 7 are graphs that show FEM simulated frequency responses forS₀ mode and type I S₁ mode Lamb wave resonators. These graphs eachinclude two curves corresponding to Lamb wave resonators with differenttypes of hammer-head IDTs for piston mode and a third curvecorresponding to Lamb wave resonator with a traditional IDT. FIG. 6 is agraph that includes a comparison of the FEM simulated frequency responsefor S0 mode Lamb wave resonators with two types of hammer-head IDTs forpiston mode according to embodiments and a traditional IDT. FIG. 7 is agraph that includes a comparison of the FEM simulated frequency responsefor type I S₁ mode Lamb wave resonators with two types of hammer-headIDTs for piston mode according to embodiments and a traditional IDT. Bycarefully designing a metal ratio, which corresponds to the reduction inphase velocity together with the border length w_(b), the amplitude ofthe transverse modes in S₀ mode and type I S₁ mode in a piston mode Lambwave resonator can be effectively reduced and/or eliminated in theelectrical response of the piston mode Lamb wave resonator.

Assuming the phase velocity difference between the gap region v_(p,g)and active region v_(p,a) is invariant, the desired optimum normalizedborder width w_(b)/λ can be proportional to the normalized velocitydifference between the border region and active region, independent fromaperture length w_(a) as reflected in Equation 5.

$\begin{matrix}{\frac{w_{b}}{\lambda} \propto {\frac{v_{p,a} - v_{p,b}}{v_{p,a}}.}} & ( {{Equation}\mspace{14mu} 5} )\end{matrix}$

FIG. 8 is a graph of a FEM simulated admittance for a piston mode devicewith a border region length that is larger than typically desired. FIG.8 shows the admittance of a S₀ mode piston mode Lamb wave resonator whenthe length wb of the border region is larger than typically desired. Inthis case, the hammer head no longer works just as the wave fieldmodification region, but as the transducer itself. Thus, anotherresonance is excited by the hammer head at frequency that is slightlylower than the main mode due to the larger duty factor. This can resultin the response being split into two resonance peaks. As shown in theinset of FIG. 8, the resonance displacement of the first peak happensright at the hammer head region and for the main mode the effectiveactive region is reduced to a length of the length of the active regionwa minus two times the length of the border region wb (i.e., to a lengthrepresented by w_(a)−2w_(b)).

For Lamb wave devices on aluminum nitride, the transverse modes arerelatively strong in the large-k²eff type I modes. The dispersioncharacteristics for the first four Lamb wave modes have been studied.Except for part of the S1 mode, most Lamb wave modes exhibit positivegroup velocity and type I dispersion. By wave vector superposition, theactive region width wa is found to determine the transverse modefrequency. The slowness curve indicates that the gap region width wg inS0 Lamb wave resonators does not play a significant role in wave guidingand energy trapping, and instead can provide a relatively small impacton transverse mode strength.

The piston mode for type I Lamb wave modes created by including a slowborder region at the edge of the active aperture can reduce and/oreliminate the transverse modes. At the same time, the slow border regionmay not introduce degradations. The FEM simulated aluminum nitride S0and type I S1 mode Lamb wave resonators using hammer head type borderregions that enable piston mode can exhibit spurious free operation.These spurious free, high Q, and large k²eff Lamb wave resonators enablesingle-chip filters, oscillators and sensors with desirable performancecharacteristics.

FIGS. 9A and 9B are diagrams of cross sections of example Lamb waveresonators. These Lamb wave resonators can be piston mode Lamb waveresonators that include IDTs in accordance with any suitable principlesand advantages disclosed herein.

FIG. 9A is a diagram of cross section of a Lamb wave resonator 30according to an embodiment. The Lamb wave resonator 30 includes featureof a SAW resonator and an FBAR. As illustrated, the Lamb wave resonator30 includes a piezoelectric layer 35, an IDT 36 on the piezoelectriclayer 35, an electrode 37, an air cavity 38, and a semiconductorsubstrate 39. The piezoelectric layer 35 can be a thin film. Thepiezoelectric layer 35 can be an aluminum nitride layer as illustrated.Alternatively, the piezoelectric layer 35 can be any other suitablepiezoelectric layer. In certain instances, the piezoelectric layer 35can be a lithium niobate layer or a lithium tantalate layer. Thefrequency of the Lamb wave resonator can be based on the geometry of theIDT 36. The electrode 37 can be grounded in certain instances. In someother instances, the electrode 37 can be floating. An air cavity 38 isdisposed between the electrode 37 and a semiconductor substrate 39. Thesemiconductor substrate 39 can be a silicon substrate as illustrated.Any suitable cavity can be implemented in place of the air cavity 38.

The Lamb wave resonator 30 includes a border region on an edge of theacoustic aperture in which the border region has a slower velocity thatan active region of the Lamb wave resonator 30. The border region cansuppress transverse modes of the Lamb wave resonator 30. The borderregion can achieve a slower velocity than the active region due tocharacteristics of the IDT 36 and/or one or more layers adjacent to theIDT 36. Example Lamb wave resonators that implement a border region thathave a slower velocity than the active region will be discussed withreference to FIG. 10A to 10J. Any suitable principles and advantages ofthese example Lamb wave resonators can be implemented in the Lamb waveresonator 30.

FIG. 9B is a diagram of cross section of an acoustic wave device 40 thatincludes a Lamb wave resonator 40 according an embodiment. The Lamb waveresonator 40 is a solidly mounted Lamb wave resonator. The Lamb waveresonator 40 is also a piston mode Lamb wave resonator. The Lamb waveresonator 40 includes feature of a SAW resonator and a solidly mountedresonator (SMR). The Lamb wave resonator 40 includes an acoustic mirrorbelow a lower electrode.

As illustrated, the Lamb wave resonator 40 includes a piezoelectriclayer 45, an IDT 36 on the piezoelectric layer 45, an electrode 47,Bragg reflectors 48, and a semiconductor substrate 39. The piezoelectriclayer 45 can be an aluminum nitride layer. Alternatively, thepiezoelectric layer 45 can be any other suitable piezoelectric layer,such as a lithium niobate (LiNO₃) layer or a lithium tantalate (LiTaO₃)layer. The frequency of the Lamb wave resonator can be based on thegeometry of the IDT 36. The electrode 47 can be grounded in certaininstances. In some other instances, the electrode 47 can be floating.The Bragg reflectors 48 function as an acoustic mirror. Any othersuitable acoustic mirror can be implemented in place of the Braggreflectors 48. Bragg reflectors 48 are disposed between the electrode 47and a semiconductor substrate 39. Any suitable Bragg reflectors can beimplemented. For example, the Bragg reflectors can be silicondioxide/tungsten (SiO₂/W). The semiconductor substrate 39 can be asilicon substrate.

The Lamb wave resonator 40 includes a border region on an edge of theacoustic aperture in which the border region has a slower velocity thatan active region of the Lamb wave resonator 40. The border region cansuppress transverse modes of the Lamb wave resonator 40. The borderregion can achieve a slower velocity than the active region due tocharacteristics of the IDT 36 and/or one or more layers adjacent to theIDT 36. Example Lamb wave resonators that implement a border region thathave a slower velocity than the active region will be discussed withreference to FIG. 10A to 10J. Any suitable principles and advantages ofthese example Lamb wave resonators can be implemented in the Lamb waveresonator 40.

Piston mode Lamb wave resonators can be implemented in a variety ofways. As an example, a metal layout of an interdigital transducer of aLamb wave resonator can contribute to a velocity in a border regionhaving a lower magnitude than a velocity in an active region. Forinstance, an end portion of an interdigital transducer electrode fingercan have wider metal than the rest of the finger. Alternatively oradditionally, a bus bar can have a lower metal coverage ratio adjacentto an end portion of an interdigital transducer finger. As anotherexample, a layer over an interdigital transducer electrode cancontribute to a velocity in a border region having a lower magnitudethan a velocity in an active region. Such a layer can be over the activeregion to increase the magnitude of the velocity in the active regionrelative to the border region. Alternatively or additionally, a layerover the border region can reduce the velocity of the border regionrelative to the active region. Example embodiments of piston mode Lambwave resonators will be discussed with reference to FIGS. 10A to 10J. Inthe Lamb wave resonators of any of FIGS. 10A to 10J, an IDT can be onaluminum nitride piezoelectric layer. Any suitable principles andadvantages of these embodiments can be combined with each other. Anysuitable principles and advantages of these embodiments can beimplemented in a piston mode Lamb wave resonator.

FIG. 10A illustrates an IDT 50 of a piston mode Lamb wave resonatoraccording to an embodiment. The IDT 50 includes fingers having hammerhead shaped end portions. The IDT 50 includes bus bars 12 and 16 and aplurality of fingers extending from the bus bars 12 and 16. Asillustrated, each of the fingers of the IDT 50 are substantially thesame. Finger 52 will be discussed as an example. Finger 52 has a bodyportion 54 that extends from bus bar 12 and an end portion 53. The endportion 53 is adjacent to the bus bar 16. The end portion 53 is widerthat the rest of the finger 52. The end portion 53 is hammer head shapedin plan view. The end portions of the fingers of the IDT 50 can make theLamb wave resonator a piston mode Lamb wave resonator.

FIG. 10B illustrates an IDT 55 of a piston mode Lamb wave resonatoraccording to another embodiment. The IDT 55 has with thicker portionsfor both border regions of each finger. The IDT 55 is like the IDT 50 ofFIG. 10A except that the fingers of the IDT 55 are wider adjacent toboth bus bars 12 and 16. Finger 56 will be discussed as an example.Finger 56 has a bus bar connection portion 59 that extends from bus bar12, a widened portion 58, a body portion 57, and an end portion 53. Boththe end portion 53 and the widened portion 58 are wider than the otherportions of the finger 56. The widened portion 58 and the end portion 53of the finger 56 are included in border regions on opposing sides of theactive region of the Lamb wave resonator that include the IDT 55. Theend portions and widened portions of the fingers of the IDT 55 can makethe Lamb wave resonator a piston mode Lamb wave resonator.

FIG. 10C illustrates an IDT 60 of a piston mode Lamb wave resonatoraccording to another embodiment. The IDT 60 includes fingers havinghammer head shaped end portions and bus bars having extension portionsadjacent to the end portions of the fingers. The IDT 60 is like the IDT50 of FIG. 10A except that the bus bars of the IDT 60 have extensionportions adjacent to end portions of fingers. Bus bars 61 and 62 eachinclude extension portions, such as extension portion 63, adjacent toend portions of fingers of the IDT 60. The Extension portions of the busbars 61 and 62 can increase the metal coverage ratio around the borderregions relative to the active region of the Lamb wave resonator. Theend portions of the finger and extension portions of the bus bars of theIDT 60 can make the Lamb wave resonator a piston mode Lamb waveresonator.

FIG. 10D illustrates an IDT 64 of a piston mode Lamb wave resonatoraccording to another embodiment. The IDT 64 has thicker end portions onborder regions of each finger and bus bars having extension portionsadjacent to end portions of the fingers. The IDT 64 includes features ofthe IDT 56 of FIG. 10C and the IDT 55 of FIG. 10B.

FIG. 10E illustrates an IDT 65 of a piston mode Lamb wave resonatoraccording to another embodiment. The IDT 65 includes fingers havingthicker end portions and thicker regions extending from a bas bar towardan active region of the Lamb wave resonator. The IDT 65 is similar tothe IDT 60 of FIG. 10C except the fingers of IDT 65 include a widenedportion extending from bus bars. As shown in FIG. 10E, finger 68 of theIDT 65 includes widened portion 66 extending from the bus bar 61 to bodyportion 57. The finger 68 also includes end portion 53.

FIG. 10F illustrates an IDT 70 of a piston mode Lamb wave resonatoraccording to another embodiment. The IDT 70 includes with bus bars 72and 74 and fingers 73 and 75 extending from the respective bus bars. Thebus bars 72 and 74 have holes 76 and 77, respectively. The holes 76 and77 are adjacent to ends of the fingers 75 and 73, respectively. Theholes can reduce a metal coverage ratio adjacent to border regions ofthe piston mode Lamb wave resonator.

FIG. 10G illustrates an IDT 80 of a piston mode Lamb wave resonatoraccording to another embodiment. The IDT 80 is like the IDT 70 of FIG.10F except that the bus bars have different holes. As illustrated inFIG. 10G, the IDT 80 includes bus bars 82 and 84 having holes 86 and 88,respectively.

FIG. 10H illustrates an IDT 90 of a piston mode Lamb wave resonatoraccording to another embodiment. The IDT 90 includes bus bars 12 and 16and fingers 92 and 95 extending from the bus bars 12 and 16,respectively. The finger 92 has thicker metal in border region portions93 and 94 that the rest of the finger 92. Similarly, the finger 94 hasthicker metal in border region portions 96 and 97 than in other portionsof the finger 94. Thicker metal can provide similar functionality aswider metal.

FIG. 10I illustrates an IDT 100 of a piston mode Lamb wave resonatoraccording to an embodiment. The IDT 100 has an oxide over border regions102A and 102B of the IDT 100. The oxide can cause a magnitude of thevelocity in the border regions to be less than the velocity in theactive region of the Lamb wave resonator. Any other suitable materialcan be included over border regions 102A and 102B to reduce themagnitude of the velocity of the border regions relative to the activeregion.

FIG. 10J illustrates an IDT 105 of a piston mode Lamb wave resonatoraccording to another embodiment. The IDT 105 has silicon nitride overactive region 106 of the IDT 105. The silicon nitride can cause amagnitude of the velocity in the active region to be greater than thevelocity in a border active region of the Lamb wave resonator. Any othersuitable material can be included over the active region 106 to increasethe magnitude of the velocity of the active region relative to theborder regions.

Lamb wave resonators can include an IDT on a piezoelectric layer andreflective gratings disposed on the piezoelectric layer on opposingsides of the IDT. The reflective gratings can reflect acoustic wavesinduced by the IDT to form a resonant cavity in such resonators. Thereflective gratings can include a periodic pattern of metal on apiezoelectric layer. FIGS. 11A to 11F are diagrams of cross sections ofLamb wave resonators with gratings. These Lamb wave resonators arepiston mode Lamb wave resonators in accordance with any suitableprinciples and advantages discussed herein. A piston mode Lamb waveresonator can be implemented with any suitable principles and advantagesof any of the Lamb wave resonators of FIGS. 11A to 11F. Although theLamb wave resonators of FIGS. 11A to 11F are free-standing resonators,any suitable principles and advantages of these Lamb wave resonators canbe applied to any other suitable Lamb wave resonators.

FIG. 11A illustrates a Lamb wave resonator 110 that includes an IDT 112,gratings 113 and 114, a piezoelectric layer 115, and an electrode 116.The IDT 112 is on the piezoelectric layer 115. In the illustrated crosssection, alternate ground and signal metals are included in the IDTs.Gratings 113 and 115 are on the piezoelectric layer 115 and disposed onopposing sides of the IDTs 112. The illustrated gratings 113 and 115 areshown as being connected to ground. Alternatively, one or more of thegratings can be signaled and/or floating. The electrode 116 and the IDT112 are on opposite sides of the piezoelectric layer 115. Thepiezoelectric layer 115 can be AlN, for example. The electrode 116 canbe grounded.

FIG. 11B illustrates a Lamb wave resonator 110′. The Lamb wave resonator110′ is like the Lamb wave resonator 110 of FIG. 11A except that theLamb wave resonator 110′ includes a floating electrode 116′.

FIG. 11C illustrates a Lamb wave resonator 110″ without an electrode ona side of the piezoelectric layer 115 that opposes the IDT 112.

FIG. 11D illustrates a Lamb wave resonator 110′″ that includes an IDT117 and gratings 118 and 119 on a second side of the piezoelectric layer115 that is opposite to a first side on which the IDT 112 and gratings113 and 114 are disposed. The signal and ground electrodes are offsetrelative to each other for the IDTs 112 and 117.

FIG. 11E illustrates a Lamb wave resonator 110″″ that includes an IDT117′ and gratings 118 and 119 on a second side of the piezoelectriclayer 115 that is opposite to a first side on which the IDT 112 andgratings 113 and 114 are disposed. The signal and ground electrodes arealigned with each other for the IDTs 112 and 117′.

FIG. 11F illustrates a Lamb wave resonator 110′ that includes an IDT117″ and gratings 118 and 119 on a second side of the piezoelectriclayer 115 that is opposite to a first side on which the IDT 112′ andgratings 113 and 114 are disposed. In the illustrated cross section, theIDT 112′ includes only signal electrodes and the IDT 117″ includes onlyground electrodes.

Lamb wave resonators can include an IDT with free edges. Suspended freeedges of a piezoelectric layer can provide acoustic wave reflection toform a resonant cavity in such resonators. FIGS. 12A to 12F are diagramsof cross sections of Lamb wave resonators with free edges. These Lambwave resonators are piston mode Lamb wave resonators in accordance withany suitable principles and advantages discussed herein. A piston modeLamb wave resonator can be implemented with any suitable principles andadvantages of any of the Lamb wave resonators of FIGS. 12A to 12F.Although the Lamb wave resonators of FIGS. 12A to 12F are free-standingresonators, any suitable principles and advantages of these Lamb waveresonators can be applied to other Lamb wave resonators.

FIG. 12A illustrates a Lamb wave resonator 120 that includes IDT 112,piezoelectric layer 115, and electrode 116. The IDT 112 is on thepiezoelectric layer 115. In the illustrated cross section, alternateground and signal electrodes are included in the IDTs. The piezoelectriclayer 115 has free edges on opposing sides of the IDT 112. The electrode116 and the IDT 112 are on opposite sides of the piezoelectric layer115. The piezoelectric layer 115 can be aluminum nitride, for example.The electrode 116 can be grounded.

FIG. 12B illustrates a Lamb wave resonator 120′. The Lamb wave resonator120′ is like the Lamb wave resonator 120 of FIG. 12A except that theLamb wave resonator 120′ includes a floating electrode 116′.

FIG. 12C illustrates a Lamb wave resonator 120″ without an electrode ona side of the piezoelectric layer 115 that opposes the IDT 112.

FIG. 12D illustrates a Lamb wave resonator 120′″ that includes an IDT117 on a second side of the piezoelectric layer 115 that is opposite toa first side on which the IDT 112 is disposed. The signal and groundelectrodes are offset relative to each other for the IDTs 112 and 117.

FIG. 12E illustrates a Lamb wave resonator 120″″ that includes an IDT117′ on a second side of the piezoelectric layer 115 that is opposite toa first side on which the IDT 112 is disposed. The signal and groundelectrodes are aligned with each other for the IDTs 112 and 117′.

FIG. 12F illustrates a Lamb wave resonator 120′ that includes an IDT117″ on a second side of the piezoelectric layer 115 that is opposite toa first side on which the IDT 112′ is disposed. In the illustrated crosssection, the IDT 112′ includes only signal electrodes and the IDT 117″includes only ground electrodes.

As discussed above, aluminum nitride Lamb wave resonators can becompatible with CMOS process technology. Accordingly, CMOS circuitry andan aluminum nitride Lamb wave resonator can be implemented on a commonsemiconductor die.

FIG. 13 is a schematic block diagram of a semiconductor die 130 thatincludes a piston mode Lamb wave resonator 132 and CMOS circuitry 134.Advantageously, the Lamb wave resonator 132 can include an aluminumnitride piezoelectric layer that can be integrated with the CMOScircuitry 134 on a common semiconductor die 130.

The piston mode Lamb wave resonators disclosed herein can be implementedin a various applications. Example applications includes in anoscillator, a sensor, and a filter. These applications will be discussedwith reference to FIGS. 14 to 16.

FIG. 14 illustrates that an oscillator 140 can include a piston modeLamb wave resonator 132. The oscillator 140 can be any oscillator thatcould benefit from a piston mode Lamb wave resonator. For example, theoscillator 140 can be included in a radio frequency front end. Theoscillator 140 can be implemented in place of another oscillator, suchas a quartz oscillator, in a variety of applications. The oscillator 140can be provide a frequency reference. The oscillator 140 can generate alocal oscillator for up converting and/or a down converting a signal.

FIG. 15 illustrates that a sensor 150 can include a piston mode Lambwave resonator 132. The sensor 150 can be any sensor that could benefitfrom a piston mode Lamb wave resonator. For example, the sensor 150 canbe arranged to sense pressure, to sense temperature, or to sense anyother suitable parameter. In some instances, the sensor 150 can beconfigured for in liquid sensing applications.

FIG. 16 illustrates that a filter 160 can include a piston mode Lambwave resonator 132. The filter 160 can receive a radio frequency signalat an input node 162 and provided a filtered radio frequency signal atan output node 164. The piston mode Lamb wave resonator 132 is coupledbetween the input node 162 and the output node 164 in the filter 160.The filter 160 is an acoustic wave filter. The filter 160 can includeany suitable number of piston mode Lamb wave resonators.

The piston mode Lamb wave resonators disclosed herein can be implementedin a variety of packaged modules. An example packaged module will now bedescribed in which any suitable principles and advantages of the pistonmode Lamb wave resonators disclosed herein can be implemented. Apackaged module can include one or more features of the packaged moduleof FIG. 17.

FIG. 17 is a schematic block diagram of a module 170 that includes afilter 172 with a piston mode Lamb wave resonator in accordance with anysuitable principles and advantage disclosed herein. The module 170includes a power amplifier 174, a switch 176, the filter 172 thatincludes a piston mode Lamb wave resonator, and an antenna switch 178.The power amplifier 174 can amplify a radio frequency signal. The switch174 can selectively electrically couple an output of the power amplifier176 to the filter 172. The filter 176 can be a band pass filter. Thefilter 176 can be included in a duplexer or other multiplexer. Theoutput of the filter 172 is coupled to an antenna switch 178. Theantenna switch 178 can be a multi-throw radio frequency switch. Theantenna switch 178 can selectively electrically couple an output of thefilter 172 to an antenna port of the module 170. The module 170 caninclude a package that encloses the illustrated elements. The filter 172with the piston mode Lamb wave resonator can be disposed on a commonpackaging substrate with the other illustrated elements of the module170. The packaging substrate can be a laminate substrate, for example.

FIG. 18 is a schematic block diagram of a wireless communication device180 that includes a filter 183 with a piston mode Lamb wave resonator inaccordance with one or more embodiments. The wireless communicationdevice 180 can be any suitable wireless communication device. Forinstance, a wireless communication device 180 can be a mobile phone,such as a smart phone. As illustrated, the wireless communication device180 includes an antenna 181, an RF front end 182, an RF transceiver 184,a processor 185, a memory 186, and a user interface 187. The antenna 181can transmit RF signals provided by the RF front end 182. The antenna181 can provide received RF signals to the RF front end 182 forprocessing.

The RF front end 182 can include one or more power amplifiers, one ormore low noise amplifiers, RF switches, receive filters, transmitfilters, duplex filters, filters of a multiplexer, filters of adiplexers or other frequency multiplexing circuit, or any suitablecombination thereof. The RF front end 182 can transmit and receive RFsignals associated with any suitable communication standards. Any of thepiston mode Lamb wave resonators disclosed herein can be implemented infilter 183 of the RF front end 182.

The RF transceiver 184 can provide RF signals to the RF front end 182for amplification and/or other processing. The RF transceiver 184 canalso process an RF signal provided by a low noise amplifier of the RFfront end 182. The RF transceiver 184 is in communication with theprocessor 185. The processor 185 can be a baseband processor. Theprocessor 185 can provide any suitable base band processing functionsfor the wireless communication device 180. The memory 186 can beaccessed by the processor 185. The memory 186 can store any suitabledata for the wireless communication device 180. The processor 185 isalso in communication with the user interface 187. The user interface187 can be any suitable user interface, such as a display.

Any of the embodiments described above can be implemented in associationwith mobile devices such as cellular handsets. The principles andadvantages of the embodiments can be used for any systems or apparatus,such as any uplink wireless communication device, that could benefitfrom any of the embodiments described herein. The teachings herein areapplicable to a variety of systems. Although this disclosure includessome example embodiments, the teachings described herein can be appliedto a variety of structures. Any of the principles and advantagesdiscussed herein can be implemented in association with RF circuitsconfigured to process signals in a frequency range from about 30 kHz to300 GHz, such as in a frequency range from about 450 MHz to 8.5 GHz.

Aspects of this disclosure can be implemented in various electronicdevices. Examples of the electronic devices can include, but are notlimited to, consumer electronic products, parts of the consumerelectronic products such as packaged radio frequency modules, uplinkwireless communication devices, wireless communication infrastructure,electronic test equipment, etc. Examples of the electronic devices caninclude, but are not limited to, a mobile phone such as a smart phone, awearable computing device such as a smart watch or an ear piece, atelephone, a television, a computer monitor, a computer, a modem, ahand-held computer, a laptop computer, a tablet computer, a microwave, arefrigerator, a vehicular electronics system such as an automotiveelectronics system, a stereo system, a digital music player, a radio, acamera such as a digital camera, a portable memory chip, a washer, adryer, a washer/dryer, a copier, a facsimile machine, a scanner, amulti-functional peripheral device, a wrist watch, a clock, etc.Further, the electronic devices can include unfinished products.

Unless the context clearly requires otherwise, throughout thedescription and the claims, the words “comprise,” “comprising,”“include,” “including” and the like are to be construed in an inclusivesense, as opposed to an exclusive or exhaustive sense; that is to say,in the sense of “including, but not limited to.” The word “coupled”, asgenerally used herein, refers to two or more elements that may be eitherdirectly connected, or connected by way of one or more intermediateelements. Likewise, the word “connected”, as generally used herein,refers to two or more elements that may be either directly connected, orconnected by way of one or more intermediate elements. Additionally, thewords “herein,” “above,” “below,” and words of similar import, when usedin this application, shall refer to this application as a whole and notto any particular portions of this application. Where the contextpermits, words in the above Detailed Description using the singular orplural number may also include the plural or singular numberrespectively. The word “or” in reference to a list of two or more items,that word covers all of the following interpretations of the word: anyof the items in the list, all of the items in the list, and anycombination of the items in the list.

Moreover, conditional language used herein, such as, among others,“can,” “could,” “might,” “may,” “e.g.,” “for example,” “such as” and thelike, unless specifically stated otherwise, or otherwise understoodwithin the context as used, is generally intended to convey that certainembodiments include, while other embodiments do not include, certainfeatures, elements and/or states. Thus, such conditional language is notgenerally intended to imply that features, elements and/or states are inany way required for one or more embodiments or that one or moreembodiments necessarily include logic for deciding, with or withoutauthor input or prompting, whether these features, elements and/orstates are included or are to be performed in any particular embodiment.

While certain embodiments have been described, these embodiments havebeen presented by way of example only, and are not intended to limit thescope of the disclosure. Indeed, the novel apparatus, methods, andsystems described herein may be embodied in a variety of other forms;furthermore, various omissions, substitutions and changes in the form ofthe methods and systems described herein may be made without departingfrom the spirit of the disclosure. For example, while blocks arepresented in a given arrangement, alternative embodiments may performsimilar functionalities with different components and/or circuittopologies, and some blocks may be deleted, moved, added, subdivided,combined, and/or modified. Each of these blocks may be implemented in avariety of different ways. Any suitable combination of the elements andacts of the various embodiments described above can be combined toprovide further embodiments. The accompanying claims and theirequivalents are intended to cover such forms or modifications as wouldfall within the scope and spirit of the disclosure.

What is claimed is:
 1. A method of filtering a radio frequency signalusing a piston mode Lamb wave resonator, the method comprising:providing the radio frequency signal to a filter that includes thepiston mode Lamb wave resonator, the Lamb wave resonator including analuminum nitride piezoelectric substrate, an interdigital transducerelectrode in contact with a first side of the aluminum nitridepiezoelectric substrate, and an electrode in contact with a second sideof the aluminum nitride piezoelectric substrate, the second sideopposing the first side; and filtering the radio frequency signal withthe filter, the filtering including generating a Lamb wave in thealuminum nitride piezoelectric substrate and suppressing a transversemode using a border region of the piston mode Lamb wave resonator, theborder region having a first velocity with a lower magnitude than asecond velocity of an active region of the piston mode Lamb waveresonator.
 2. The method of claim 1 wherein a metal layout of theinterdigital transducer electrode of the piston mode Lamb wave resonatorcontributes to the first velocity having the lower magnitude than thesecond velocity.
 3. The method of claim 1 wherein a layer over at leasta portion of the interdigital transducer electrode contributes to thefirst velocity having the lower magnitude than the second velocity. 4.An acoustic wave filter with a piston mode Lamb wave resonator, theacoustic wave filter comprising: an input node configured to receive aradio frequency signal; an output node; and a piston mode Lamb waveresonator coupled between the input node and the output node, the pistonmode Lamb wave resonator including a piezoelectric layer, aninterdigital transducer electrode on the piezoelectric layer, and an aircavity on an opposite side of the piezoelectric layer than theinterdigital transducer electrode; the piston mode Lamb wave resonatorbeing configured to generate a Lamb wave in the piezoelectric layer, thepiston mode Lamb wave resonator having an active region and a borderregion, the border region having a first velocity with a lower magnitudethan a second velocity of the active region, the border regionconfigured to suppress a transverse mode, and the acoustic wave filterbeing configured to filter the radio frequency signal.
 5. The acousticwave filter of claim 4 wherein the border region has a larger metalcoverage ratio than the active region.
 6. The acoustic wave filter ofclaim 4 wherein the interdigital transducer electrode includes a bus barand fingers extending from the bus bar, a finger of the fingersincluding an end portion opposite the bus bar that has metal that iswider than other portions of the finger.
 7. The acoustic wave filter ofclaim 4 wherein the Lamb wave resonator includes a layer over theinterdigital transducer electrode that contributes to the first velocityhaving the lower magnitude than the second velocity.
 8. The acousticwave filter of claim 4 wherein the piezoelectric layer includes analuminum nitride layer.
 9. A piston mode Lamb wave resonator withtransverse mode suppression, the piston mode Lamb wave resonatorcomprising: a piezoelectric layer; an interdigital transducer electrodein contact with the piezoelectric layer; and an electrode in contactwith the piezoelectric layer and positioned on an opposite side of thepiezoelectric layer than the interdigital transducer electrode, thepiston mode Lamb wave resonator having an active region and a borderregion, the border region having a first velocity with a lower magnitudethan a second velocity of the active region, the border regionconfigured to suppress a transverse mode, and the piston mode Lamb waveresonator configured to generate a Lamb wave in the piezoelectric layer.10. The piston mode Lamb wave resonator of claim 9 wherein thepiezoelectric layer is an aluminum nitride layer.
 11. The piston modeLamb wave resonator of claim 9 wherein the interdigital transducerelectrode includes a bus bar and a plurality of fingers extending fromthe bus bar, each of the fingers having an end portion opposite the busbar.
 12. The piston mode Lamb wave resonator of claim 11 wherein the endportions of the fingers include metal that is wider than other portionsof the respective fingers.
 13. The piston mode Lamb wave resonator ofclaim 11 further comprising an oxide over the end portions of thefingers.
 14. The piston mode Lamb wave resonator of claim 11 furthercomprising silicon nitride over a portion of the interdigital transducerelectrode, the end portions being free from the silicon nitride.
 15. Thepiston mode Lamb wave resonator of claim 11 wherein the interdigitaltransducer electrode includes a second bus bar having a lower metalcoverage ratio adjacent the end portions than in other portions of thesecond bus bar.
 16. The piston mode Lamb wave resonator of claim 9wherein the border region has a larger metal coverage ratio than theactive region.
 17. The piston mode Lamb wave resonator of claim 9wherein the border region has a larger metal coverage ratio than aninactive region of the piston mode Lamb wave resonator.
 18. The pistonmode Lamb wave resonator of claim 9 further comprising reflectivegratings on opposing sides of the interdigital transducer electrode. 19.The piston mode Lamb wave resonator of claim 9 wherein the piston modeLamb wave resonator has free edges.
 20. The piston mode Lamb waveresonator of claim 9 further comprising an air cavity, the electrodepositioned between at least a portion of the air cavity and thepiezoelectric layer.
 21. The piston mode Lamb wave resonator of claim 9further comprising an acoustic mirror, the electrode positioned betweenat least a portion of the acoustic mirror and the piezoelectric layer.22. The piston mode Lamb wave resonator of claim 9 wherein the pistonmode Lamb wave resonator is arranged so as to have even order modes thathave a multiple of full wave lengths in the active region.